This invention relates to a method of controlling an internal combustion engine, and more particularly to a method of controlling an internal combustion engine which is capable of setting the timing of closing an intake valve or intake valves as desired.
In such an internal combustion engine which is capable of setting the timing of closing an intake valve or intake valves as desired, it is conventionally known that by varying intake valve-closing timing (.theta..sub.IC) with the engine rotational speed being held constant and the ignition timing being kept optimum, there can be found valve-closing timing (hereinafter referred to as "the optimum timing" .theta..sub.BIC at which the maximum output of the engine is obtained as shown in FIG. 13 (a). FIG. 13 (b) shows curves of this characteristic of valve-closing timing obtained with the engine rotational speed Ne set to different values. As indicated by a solid line curve A in FIG. 13 (b), the optimum timing .theta..sub.BIC varies depending on the engine rotational speed Ne.
Therefore, if the intake valve-closing timing is held at .theta..sub.ICO as indicated by the broken line in the same figure, the optimum timing is obtained at Ne=Ne4. However, at other engine rotational speeds, the maximum output of the engine cannot be obtained. In view of this fact, an internal combustion engine which is capable of changing the valve-closing timing between two points (.theta..sub.IC1 and .theta..sub.IC2) has been proposed by Japanese Patent Publication (Kokoku) No. 49-33289. However, in such an engine as well, as indicated by one-dot-chain lines in the figure, it is impossible to obtain the maximum engine output at all engine rotational speeds.
On the other hand, there is a problem that knocking is more liable to occur as larger engine output is obtained. In order to solve this problem, an anti-knocking system for an engine which is capable of changing intake-valve closing timing, has been proposed by Japanese Provisional Utility Model Publication (Kokai) No. 59-159705, in which when knocking is detected, the intake valve-closing timing is changed.
However, according to this system, when knocking ceases to be detected, the intake valve-closing timing is restored to the former timing, so that in the case where a regular gasoline is used for an engine for which a gasoline having a high octane number is suitable, there arises a problem of occurrence of hunting, i.e. a repeated sequence of occurrence of knocking--changing of intake valve-closing timing--ceasing of knocking--restoration of intake valve-closing timing--re-occurrence of knocking.